Proposed resolution:

This wording is relative to N4713.

1. Modify [syserr.errcode.overview] as indicated:

   namespace std {
     class error_code {
     public:
       // [syserr.errcode.constructors], constructors
       […]
       error_code(int val, const error_category& cat) noexcept;
       error_code(int val, const error_category&& cat) = delete;
       […]
       // [syserr.errcode.modifiers], modifiers
       void assign(int val, const error_category& cat) noexcept;
       void assign(int val, const error_category&& cat) = delete;
       […]
     };
     […]
   }
   

[ 2018-06-18 after reflector discussion ]

Priority set to 3

Constructor error_code(int val, const error_category& cat) and member function void assign(int val, const error_category& cat) could be misused if a custom error_category is provided:

error_code ec{1, test_category{}}; // ec holds a pointer/reference to a temporary